Many types of modern gas turbine engines are of the mixed flow variety wherein a primary fluid stream is mixed with a secondary fluid stream prior to discharge of the exhaust fluid into the atmosphere as a common thrust-producing mixed flow fluid stream. Generally, the primary fluid stream is the high velocity, high temperature exhaust effluent flowing from the turbine stage and the secondary fluid stream is air or gas at a lower temperature and velocity, e.g. air from the engine fan stage. As is known in the art, such mixed flow has two beneficial effects. First, engine thrust is improved since the mixed gases have a higher mass-velocity product that that of the turbine exhaust gases alone. Secondly, the noise level is reduced since the exhausted mixed gases have a lower velocity than the velocity of the turbine exhaust gases.
Decreasing the velocity of the gaseous thrust-producing fluid stream reduces the overall engine noise by decreasing that component of engine noise commonly called jet noise. Jet noise is not generated within the gas turbine engine, but is caused by turbulence resulting from the large velocity gradients that exist at the boundary between the exhausted fluid stream and the atmosphere. Since the intensity of the jet noise component is exponentially related to the velocity of the exhausted fluid, decreasing the fluid velocity prior to discharge into the atmosphere substantially reduces the intensity of the jet noise.
One prior art arrangement for mixing the two fluid streams includes an outer duct of substantially circular cross-section coaxially mounted about a generally tubular mixer section. In many instances, the mixer section coaxially surrounds an engine plug which extends rearwardly along the axial center line of the gas turbine engine. In such an arrangement the turbine exhaust gases flow through the annular duct formed between the engine plug and the inner surface of the mixer section and the cooler, lower velocity gases, e.g. fan air, flow through the annular duct formed between the inner surface of the outer duct and the outer surface of the mixer section. The high temperature, high velocity turbine exhaust mixes with the low temperature, low velocity gases as the two fluid streams flow past the mixer section exit plane.
One type of the above-described mixing apparatus, commonly called a daisy mixer, includes a mixer section having a plurality of axially extending circumferentially spaced lobes or corrugations of increasing radial dimension relative to the mixer length. Effectively, these lobes increase the peripheral length of the mixing boundary formed at the mixer section exit plane to thereby provide more efficient mixing, and hence, lower jet noise.
Such prior art daisy type mixers are employed within jet engine exhaust nozzles wherein an exhaust nozzle is mounted on the aft portion of the gas turbine engine for mixing the engine exhaust with ambient air or air supplied by various engine components such as the compressor stage or fan stage of a turbofan engine. Additionally, such daisy type mixers are often utilized within a bypass type turbofan gas turbine engine in which a portion of the air supplied by a forward mounted fan stage is ducted around the compressor and combustor stages and mixed with the turbofan exhaust gases at a position aft of the engine turbine stages.
Although prior art daisy mixers are effective in reducing the overall jet noise by reducing the level of the jet noise component, the several other noise components produced by the engine generally pass through the mixer and out the engine tail pipe or exhaust orifice substantially unaltered. One such noise component, due to acoustic energy contained within the high temperature, high velocity turbine exhaust gases is commonly called core noise. Although the exact source mechanisms of core noise are unknown, core noise includes contributions from the combustion of the jet fuel within the engine combustor stage and contributions due to the interaction of the unstable combustor gases with the downstream blades. Further, it is known that core noise is distributed over a relatively low frequency portion of the frequency spectrum, commonly peaking at between 200 and 600 Hz regardless of engine size.
A second noise component that is not substantially altered by prior art daisy mixers is commonly called aft fan noise. Aft fan noise results from unsteady or turbulent forces on the fan blades due to unsteady flow conditions within the fan stage and also results from shock waves produced by the leading edge of the fan rotors. The acoustic energy which constitutes the fan noise occupies a portion of the frequency spectrum generally between 2 and 6 KHz.
In prior art gas turbine engines, attempts to reduce or suppress fan noise have generally included design of the fan structure to minimize the generation of fan noise and also attempts to acoustically treat the fan ducts to attenuate the noise energy as it propagates through the fan duct. With respect to core noise, the prior art attempts have generally been limited to muffler-like structure that is mounted in an extended exhaust duct to attenuate the core noise as it passes from the engine. None of the prior art attempts have been totally successful and much of the prior art presents serious drawbacks in that improved noise performance is achieved only by accepting substantial added weight and engine performance penalties.
Accordingly, it is an object of this invention to provide means for suppressing the noise generated by a gas turbine engine.
It is another object of this invention to provide a mixer assembly for use within a mixed flow gas turbine engine which suppresses internally generated noise components as well as the externally generated jet noise component.
It is yet another object of this invention to provide a daisy type gas turbine mixer assembly for use in mixing fan air with turbine exhaust gases wherein the mixer is arranged to attenuate the core noise associated with the turbine exhaust gases and to also attenuate the aft fan noise associated with the air flowing from the fan stage.
It is still another object of this invention to provide a mixer assembly for use in a mixed flow gas turbine engine with the mixer assembly including integral means for the suppression of core noise and fan noise to thereby improve noise performance without substantial increase in engine weight or appreciable loss of engine performance.